Metal-organic materials for CO2 adsorption

ABSTRACT

Embodiments of the present disclosure provide for metal-organic materials (MOMs), systems that exhibit permanent porosity and using hydrophobic MOMs to separate components in a gas, methods of separating CO 2  from a gas, and the like.

BACKGROUND

Metal-organic framework (MOF) materials that exhibit permanent porosityhave received extensive interest due to their potential applications forgas storage or capture. However, many of the currently used MOFs havelimitations, and thus, other types of MOFs having desiredcharacteristics are needed to be used in certain applications.

SUMMARY

Embodiments of the present disclosure provide for metal-organicmaterials (MOMs), systems using MOMs to separate components in a gas,methods of separating CO₂ from a gas, and the like.

An embodiment of the method of capturing CO₂ in a gas, among others,includes: exposing the gas to a metal-organic material (MOM), whereinthe gas includes CO₂ at a concentration in the gas of about 5% or less;and capturing the CO₂ in the MOM.

An embodiment of the system for capturing CO₂ in a gas mixture, amongothers, includes: a first structure including a metal-organic material(MOM), wherein the gas includes CO₂ at a concentration in the gas ofabout 5% or less; and a second structure for introducing the gas to thefirst structure, wherein CO₂ is removed from the gas after the exposureto the MOM to form a modified gas (CO₂ free), wherein the secondstructure flows the modified gas away from the first structure.

An embodiment of the method of separating components in a gas mixture,among others, includes: exposing a gas including a CO₂ and a secondcomponent to a metal-organic material (MOM), wherein the MOM has agreater relative affinity (kinetic and thermodynamic) for the CO₂ over asecond component, wherein the CO₂ is at a concentration in the gas ofabout 5% or less; and capturing the first component in the MOM.

An embodiment of the system for separating components in a gas, amongothers, includes: a first structure including a metal-organic material(MOM), wherein the gas includes CO₂ and a second component, wherein theMOM has a greater relative affinity for CO₂ over the second component,wherein the CO₂ is at a concentration in the gas of about 5% or less;and a second structure for introducing the gas to the first structure,wherein CO₂ is removed from the gas after the exposure to the MOM toform a modified gas, wherein the second structure flows the modified gasaway from the first structure.

An embodiment of the composition, among others, includes: a MOMcomprising [Cu(pyr)₂(SiF₆)]_(n), wherein n is 1 to 100,000,000, andwherein the MOM has a pore size of about 3.5 Å.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosed devices and methods can be betterunderstood with reference to the following drawings. The components inthe drawings are not necessarily to scale, emphasis instead being placedupon clearly illustrating the relevant principles. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 is a schematic that illustrates the ability of pore size tuningin the channel structures of SIFSIX-2-Cu-i, SIFSIX-3-Zn or/andSIFSIX-3-Cu. In particular, a) SIFSIX-2-Cu-i; pores size 5.15 Å, BETapparent surface area (N₂ adsorption) 735 m²g⁻¹; b) SIFSIX-3-Zn; poressize 3.84 Å, BET apparent surface area (determined from CO₂ adsorptionisotherm) 250 m²g⁻¹; and c) SIFSIX-3-Cu; pores size 3.3.5 Å, BETapparent surface area (determined from CO₂ adsorption isotherm) 300m²g⁻¹. Color code: Dipyridilacetylene (dpa, thick light greypolyhedral), pyrazine (pyz, medium grey polyhedral), Zn, Cu (darker greypolyhedral), Si (light grey spheres), F (light grey spheres). All guestmolecules are omitted for clarity. Note that the grey net represents theinterpenetrated net in SIFSIX-Cu-2-i.

FIG. 2A illustrates the variable temperature adsorption isotherms of CO₂for SIFSIX-Cu-3). FIG. 2B illustrates the pore size distribution fromCO₂ sorption isotherms at 77 K for SIFSIX-Cu-3.

FIG. 3A illustrates the CO₂ volumetric uptake on SIFSIX-Cu-3,SIFSIX-Zn-3 and SIFSIX-Cu-2-i as compared to Mg-MOF-74. FIG. 3Billustrates the isosteric heats of adsorption at low coverage forSIFSIX-Cu-3, SIFSIX-Zn-3 and SIFSIX-Cu-2-i.

FIG. 4A illustrates the column breakthrough test of CO₂/N₂:1000ppm/99.99% for SIFSIX-Cu-3, SIFSIX-Zn-3 in dry condition. FIG. 4Billustrates the column breakthrough test of CO₂/N₂:1000 ppm/99.99% forSIFSIX-Cu-3 in dry as well as at 74% RH.

FIG. 5 illustrates a comparison of PXRD diagrams of SIFSIX-3-Cu withother SIFSIX-pyz MOFs (λ_(Cu)=1.5406 nm).

FIG. 6 illustrates a Lebail fit for SIFSIX-3-Cu (λ_(Cu)=1.5406 nm).

FIG. 7 illustrates a comparison of calculated and experimental PXRDdiagrams for SIFSIX-3-Cu (λ_(Cu)=1.5406 nm).

FIG. 8 illustrates a comparison of the metal environment in SIFSIX basedMOFs through the metal-nitrogen (top) and metal-fluorine (bottom)distances.

FIG. 9 illustrates variable temperature adsorption isotherms forSIFISX-2-Cu-i.

FIG. 10 illustrates variable temperature adsorption isotherms forSIFISX-3-Zn.

FIG. 11 illustrates a CO₂ adsorption isotherms at very low pressures upto 0.25 bar (250 mbar) for SIFSIX-Cu-3, SIFSIX-Zn-3 and SIFSIX-Cu-2-i ascompared to Mg-MOF-74[J. Am. Chem. Soc. 130, 10870-10871 (2008)] andUTSA-16 [Nat. Commun. 3:954 doi: 10.1038/ncomms1956 (2012)].

FIG. 12 illustrates competitive adsorption kinetics of CO₂/N₂:10/90 gasmixture as compared to the kinetics of adsorption of pure CO₂ at 1 barand 298 K.

FIG. 13 illustrates a column breakthrough test of CO₂/N₂:1000 ppm/99.99%for SIFSIX-Zn-3 in dry as well as at 74% RH.

FIG. 14 illustrates adsorption isotherms of CO₂, N₂ and O₂ on Sif6-3-cuat 298 K.

FIG. 15 illustrates a comparison of experimental PXRD diagrams forSIFSIX-3-Cu (λ_(Cu)=1.5406 nm) as prepared (bottom curve), after highpressure experiments and exposed to air (middle curve) and afterbreak-through experiments under dry and humid conditions (top curve).

DISCUSSION

Before the present disclosure is described in greater detail, it is tobe understood that this disclosure is not limited to particularembodiments described, as such may, of course, vary. It is also to beunderstood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting, since the scope of the present disclosure will be limited onlyby the appended claims.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit (unlessthe context clearly dictates otherwise), between the upper and lowerlimit of that range, and any other stated or intervening value in thatstated range, is encompassed within the disclosure. The upper and lowerlimits of these smaller ranges may independently be included in thesmaller ranges and are also encompassed within the disclosure, subjectto any specifically excluded limit in the stated range. Where the statedrange includes one or both of the limits, ranges excluding either orboth of those included limits are also included in the disclosure.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this disclosure belongs. Although any methods andmaterials similar or equivalent to those described herein can also beused in the practice or testing of the present disclosure, the preferredmethods and materials are now described.

As will be apparent to those of skill in the art upon reading thisdisclosure, each of the individual embodiments described and illustratedherein has discrete components and features which may be readilyseparated from or combined with the features of any of the other severalembodiments without departing from the scope or spirit of the presentdisclosure. Any recited method can be carried out in the order of eventsrecited or in any other order that is logically possible.

Embodiments of the present disclosure will employ, unless otherwiseindicated, techniques of chemistry, organic chemistry, organometallicchemistry, coordination chemistry and the like, which are within theskill of the art. Such techniques are explained fully in the literature.

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how toperform the methods and use the compositions and compounds disclosed andclaimed herein. Efforts have been made to ensure accuracy with respectto numbers (e.g., amounts, temperature, etc.), but some errors anddeviations should be accounted for. Unless indicated otherwise, partsare parts by weight, temperature is in ° C., and pressure is at or nearatmospheric. Standard temperature and pressure are defined as 25° C. and1 atmosphere.

Before the embodiments of the present disclosure are described indetail, it is to be understood that, unless otherwise indicated, thepresent disclosure is not limited to particular materials, reagents,reaction materials, manufacturing processes, or the like, as such canvary. It is also to be understood that the terminology used herein isfor purposes of describing particular embodiments only, and is notintended to be limiting. It is also possible in the present disclosurethat steps can be executed in different sequence where this is logicallypossible.

It must be noted that, as used in the specification and the appendedclaims, the singular forms “a,” “an,” and “the” include plural referentsunless the context clearly dictates otherwise. Thus, for example,reference to “a support” includes a plurality of supports. In thisspecification and in the claims that follow, reference will be made to anumber of terms that shall be defined to have the following meaningsunless a contrary intention is apparent.

General Discussion

Embodiments of the present disclosure provide for metal-organicmaterials (MOMs), systems using MOMs to separate components in a gas,methods of separating CO₂ from a gas, and the like. In an embodiment,the MOM can be used to remove or separate CO₂ in a gas at a variety ofconcentrations. In a particular embodiment, the MOM can be used toremove or separate trace levels of CO₂ in a gas (e.g., CO₂ at aconcentration of about 5% or less in a gas) with high specificity andremoval capacity, which can be advantageous in a number of industries.

The growing interest in combatting the greenhouse gas effect triggered arising interest in the Direct Air Capture (CAD) as a viable option toreduce greenhouse gases emissions in uniform way. Although morechallenging than post-combustion capture, embodiments of the presentdisclosure can be used in CAD since embodiments of the MOM have suitableuptake, kinetics, energetics and CO₂ selectivity, which enables removalor separation of trace levels of CO₂ concentration.

Removal of trace amounts of CO₂ from air for industrial purposes isimportant particularly for pre-purification of air when atmospheric airis used during the separation of nitrogen and oxygen. In a particularsystem, prior to air separation using cryogenic distillation or pressureswing adsorption (PSA), air must be CO₂ free to avoid (i) blockage ofheat-exchange equipment due to frozen CO₂ during the liquefactionprocess and (ii) adsorbents (e.g., zeolites) contamination used foroxygen production by PSA. MOMs of the present disclosure can be used toremove CO₂ for air separation using cryogenic distillation or PSA.

In another application, alkaline Fuel Cells (AFCs) require the oxygenand hydrogen used as feedstock to be CO₂ free as trace amounts of CO₂(300 ppm) degrade the electrolyte in AFCs. In this regard, MOMs of thepresent disclosure can be used in AFCs to remove CO₂.

Efficient removal of CO₂ at low concentrations is also vital for theproper operation of breathing systems in confined spaces such assubmarines, planes, aerospace shuttles, and the like. In particular, inlong-term space flight and submarine missions, CO₂ must be removed fromthe air and recycled because resupply opportunities are less frequent ornon-existent. Humans require oxygen, and in return emit carbon dioxide.An average crew member requires approximately 0.84 kg of oxygen andemits approximately 1 kg of carbon dioxide. As a result, the ability tocontinuously purify the exhaled air (with a CO₂ concentration of 5% orless) can lead to an optimal recycling and considerable reduction infresh air supply in closed, confined spaces such as space shuttles,planes and submarines. In this regard, embodiments of the MOMs can beused in breathing systems to manage CO₂ concentration.

Efficient CO₂ removal is also of importance in mining and rescuemissions and diving. Thus, embodiments of the MOMs can be used to manageCO₂ concentration.

CO₂ removal is also a concern in medical applications such as anesthesiamachines. The use of anesthesia machines is a growing clinical trendthroughout the world, driven by the need to reduce cost and improvepatient care. CO₂ removal in anesthesia machines is particularlyimportant in semi- or closed rebreathing systems in which therebreathing fraction is at least 50% of the exhaled gas volume isdirected back to the patient after proper carbon dioxide removal in thenext exhalation. Current solutions are costly, have limitedrecyclability, and have large disposal costs associated with them, whileMOMs of the present disclosure can be used to remove and manage CO₂concentration levels in anesthesia machines and in some situations canhave a lifetime that is 10,000 times greater than current standards.

In an embodiment, the MOM can be porous and can be a three dimensionalnet so that molecules can be disposed (e.g., captured) within (e.g.,pores or cavities) the MOM to the exclusion of other molecules. In anembodiment, the MOM combines sorption thermodynamics and kinetics toachieve advantageous results. Embodiments of the present disclosuredescribe MOMs that have no unsaturated metal centers and the pore wallshave no hydrogen bonding donors or acceptors, while having strongelectrostatics for inducing dipoles in polarizable molecules such asCO₂. For example, a gas such as CO₂ is absorbed faster and stronger thanother gases in the gas mixture, so that CO₂ can be captured in the MOMsto the substantial exclusion of the other gases. In particular, the MOMcan be used to remove or separate CO₂ in a gas, where the gas includesCO₂ at a concentration of about 5% or less. In an embodiment of thepresent disclosure have enhanced CO₂ interactions at the same time havereduced interactions with water vapor.

In an embodiment, the MOM can be a hydrophobic MOM. In an embodiment,the hydrophobic MOM can be used to separate CO₂ from one or more othergases, where the gas includes water vapor. Due to its hydrophobiccharacteristic, hydrophobic MOMs can be used in methods and systems thatuse gases that include water vapor, which was not previously possible inporous materials that exhibit strong physisorption towards CO₂. This isadvantageous because other systems and methods that use other MOMs orother porous materials must separate water vapor from the gas prior tothe gas being introduced to the other MOMs or porous materials since theother MOMs or porous materials have a higher affinity for the watervapor than CO₂. If the water vapor is not removed, the other MOMs arenot effective at removing CO₂. In addition, MOMs of the presentdisclosure can remove trace levels of CO₂. Embodiments of the systemsand methods can be simplified and result in reduced expenditure sincethe water vapor does not have to be removed prior to introduction to thehydrophobic MOMs. Even in the presence of water vapor, hydrophobic MOMsused in embodiments of the present disclosure are still effective atremoving CO₂ and are highly selective in separating CO₂ from other gasessuch as N₂, H₂, and/or CH₄, even at trace levels of CO₂.

In particular, embodiments of the present disclosure can be used in CO₂capture systems where the gas has trace levels of CO₂, and these caninclude direct air capture systems, pre-purification systems (e.g., PSAand cryogenic distillation), AFCs, breathing systems, mining and rescuemissions, diving, and in medical applications. In addition, embodimentsof the present disclosure can be used in post-combustion systems (e.g.,flue gas to separate CO₂ and N₂), pre-combustion systems (e.g., shiftedsynthesis gas stream to separate CO₂ and H₂), and/or natural gasupgrading (e.g., natural gas cleanup to separate CO₂ and CH₄). In anembodiment, the hydrophobic MOMs can be used to separate other gases andcan be used in processes such as He separation from natural gas, Arseparation, Kr separation, and H₂/D₂ separation.

Embodiments of the present disclosure provide for MOMs that are threedimensional nets that have a primitive cubic topology (See FIG. 1) thatcan be used in methods and systems of the present disclosure. In anembodiment, the MOM (e.g., [Cu(pyr)₂(SiF₆)]_(n)) can be designed andsynthesized using two dimensional square grids (or nets) (e.g., Cu(4,4′-dipyridylacetylene)₂) that are linked via metal nodes using apillar (e.g., SiF₆ ²⁻). In an embodiment, the two dimensional squaregrids include metal cations, metal cluster molecular building blocks(MBBs), or metal-organic polyhedral supermolecular building blocks(SBBs). The MBBs or SBBs serve the geometric role of the node in anetwork and they are connected by organic molecules, inorganic anionsand/or metal complexes, which serve as linkers. The two dimensionalsquare grids are connected to one another using other linkers or pillarsthat connect the metal nodes. In an embodiment, the components of theMOM (the two dimensional square grids, and its components, and pillars)can be selected to design a MOM that can be used in a system or methodthat includes trace levels of CO₂ and/or water vapor and is highlyeffective at separating gases due to the MOM having a higher relativeaffinity for one component of the gas (e.g., CO₂) over one or more othercomponents (e.g., N₂, H₂, and CH₄) in the gas. In this way not only isthe MOM able to operate in methods and systems having high water vaporconditions, but the MOM is highly selective between or among CO₂ andother components, even where the CO₂ concentration is at trace levels.

In an embodiment, a method of the present disclosure includes exposing agas to a MOM (e.g., [Cu(pyr)₂(SiF₆)]_(n)). As noted above, the MOM has agreater relative affinity for a first component (e.g., trace levels ofCO₂) of the gas over a second component of the gas. The phrase “greaterrelative affinity” or similar phrases mean that a MOM can interact witha first component much more strongly than a second component so that theMOM and the first component interact to the substantial exclusion of thesecond component. Thus, the first component can be captured (e.g.,separated) from the gas mixture to form a modified gas, where themodified gas includes the second component and a substantially reducedamount (e.g., greater than about 80% or more, about 90% or more, about95% or more, about 99% or more, about 99.9% or more, removal of thefirst component from the gas) of the first component.

In an embodiment, the selectivity for CO₂/N₂ can be about 100 or more,about 500 or more, about 1000 or more, or about 2000 or more, based oncolumn breakthrough experiments and at conditions of 25° C. and 1 bartotal pressure using gas stream with CO₂ in the range 400 ppm to 50%.The column breakthrough tests were run by passing the non-treated gasstream trough a column containing the MOFs. The gas downstream thecolumn is monitored by a gas analyzer to determine the change incomposition of each gas.

In an embodiment, the MOM has a removal capacity of about 1.2-2 mmol/g(44-71 cm³ (STP)/cm³) at 400 ppm or about 2.2-2.5-mmol/g (80-88 cm³(STP)/cm³) at 5000 ppm.

In an embodiment, the system for capturing CO₂ in a gas mixture canincludes pressure (vacuum) swing adsorption, temperature swingadsorption, and combination thereof. In an embodiment, the method ofseparating components in a gas mixture can include removal of high CO₂concentrations, removal of intermediate CO₂ concentrations, and very lowCO₂ concentration. In an embodiment, the method of capturing CO₂ in agas can include bulk CO₂ separation (50% CO₂>), CO₂ purification (<5%)and CO₂ ultra-purification (<1%).

As described herein, a substantial advantage of some embodiments of thepresent disclosure is that methods and systems using the MOMs can beconducted using a gas having water vapor, which is a completelyunexpected result since most of other MOMs and related inorganic porousmaterials are typically hydrophilic and have a strong affinity for waterso that the water vapor needs to be substantially or completely removedfrom the gas for the MOM to be commercially viable. In an embodiment,the water vapor in the gas can be at a concentration of about 1% to 10%at a temperature of about 273K to 340K.

In an embodiment, the gas can include two or more components and caninclude water vapor. In an embodiment, gas does not include water vapor.It should be noted that in many situations, the gas may primarilyinclude a few components or only a few components that are important tothe desired separation. In an embodiment, the component can include oneor more of the following: CO₂ (e.g., trace levels), N₂, H₂, CH₄, He,hydrocarbons having 2 or more carbons (saturated or unsaturated and/orlinear or branched), and a combination thereof. In an embodiment, CO₂can be in the gas in an amount of about 400 ppm to 50% or in an amountof about 5% or less. In an embodiment, N₂ can be in the gas in an amountof about 50% to 99.99%. In an embodiment, H₂ can be in the gas in anamount of about 50% to 99.99%. In an embodiment, CH₄ can be in the gasin an amount of about 50% to 99.99%. In an embodiment, He can be in thegas in an amount of about 50% to 99.99%.

In an embodiment, the components in a gas can be separated using asystem to introduce the gas to the MOM and remove the modified gas. Inan embodiment, a first structure or device including the MOM can beinterfaced with a second structure or device to introduce a gas to thefirst structure so that the gas and the MOM can interact so that the MOMcan capture the first component (e.g., trace levels of CO₂). After asufficient period of time and under appropriate temperature conditions,the remaining gas or modified gas can be removed from the firststructure. This process can be repeated as appropriate for theparticular system. After a period of time, the first component can beremoved from the MOM and the MOM can be reused and/or recycled using anappropriate gas handling system.

In an embodiment, the first structure and the second structure caninclude those used in systems such as direct air capture systems,pre-purification systems (e.g., PSA and cryogenic distillation), AFCs,breathing systems, mining and rescue missions, diving, medicalapplications, post-combustion systems, pre-combustion systems, naturalgas upgrading systems, and He separation systems. In particular, thefirst structure can include structures such as those used in typicalsystems mentioned above. In an embodiment, the second structure caninclude standard gas handling systems, valves, pumps, flow meters, andthe like.

As noted above, MOMs can be three dimensional nets that can have aprimitive cubic topology but they could also exhibit a differenttopology (See FIG. 1). In an embodiment, the MOM can be designed andsynthesized using two dimensional square nets that are linked via metalnodes using a molecule or ion that serves the role of a pillar. In anembodiment, the two dimensional square nets can include metal cations,MBBs, or SBBs, and linkers can be used to bond the metal ions and theMBB and the SBB.

In an embodiment, MOMs can have one of the following generic structure:(M(L)_(a)(P)_(n)), where M is the metal ion, L is the linker, and P isthe pillar, a is 2 and n is 1. In an embodiment, the MOM has a pore sizeof about 3.3 Å to 3.9 Å or about 3.5 Å. L and P can be difunctionalligands that are capable of linking the metal clusters or ions such aspyrazine, 4,4′-bipyridine, 1,4-benzenedicarboxylate, hexaflourosilicate,and hexaflourotitanate. In an embodiment, these types of MOMs aredescribed in references 13-15 below in the Example, which areincorporated herein by reference for how to describe MOMs and MOFs andthe components of each.

In an embodiment, the metal cations can include M¹⁺ (e.g., Na, K, Li,Ag, etc.); M²⁺ (e.g., Cu, Zn, Co, Mn, Mo, Cr, Fe, Ca, Ba, Cs, Pb, Pt,Pd, Ru, Rh, Cd, etc.); M³⁺ (e.g. In, Fe, Y, Ln (Yb, Tb, etc.)); M⁴⁺(e.g., Zr, Ti, V, etc.); or other higher oxidative state metals such as+4, +5, +6, +7, and +8. In an embodiment, the MBBs and SBBs can includethese metal cations as well.

In an embodiment, the linkers in the two dimensional square grid caninclude organic molecules, inorganic anions and/or metal complexes. Inan embodiment, the linkers can include pyrazine (substituted andunsubstituted) and derivatives thereof, bipyridine (substituted andunsubstituted) and derivatives thereof, and the like.

In an embodiment, the pillars can include organic molecules, inorganicanions and/or metal complexes. In an embodiment, the pillars can includeSiF₆ ²⁻, GeF₆ ²⁻, TiF₆ ²⁻, SnF₆ ²⁻, PF₆ ⁻, and NO₃ ⁻.

In an embodiment, MOMs of the present disclosure can be designedconsistent with the description of (M(L)_(a)(P)_(n)) so that the MOM hasa pore size of about 3.3 Å to 3.9 Å or about 3.5 Å. As described in theExample, MOMs having pore sizes in this range and having a high chargedensity are effective at trapping CO₂ at concentrations of about 5% orless, about 4% or less, about 0.1 to 5%, or about 0.1 to 4%. In anembodiment, the MOM can include: [Cu(pyr)₂(SiF₆)]_(n), wherein n is 1 to100,000,000.

EXAMPLE

Now having described the embodiments of the present disclosure, ingeneral, the Examples describe some additional embodiments of thepresent disclosure. While embodiments of present disclosure aredescribed in connection with the Examples and the corresponding text andfigures, there is no intent to limit embodiments of the presentdisclosure to these descriptions. On the contrary, the intent is tocover all alternatives, modifications, and equivalents included withinthe spirit and scope of embodiments of the present disclosure.

Example 1 Brief Introduction

Direct air capture (DAC) is another alternative approach to mitigate theincreasing CO₂ emissions while accounting for both carbon emissions fromvarious sources such as transportation sector and stationary powerplants sources. We previously we reported how a material design andengineering approaches to pore size control, in combination withsuitable sorption energetics of favourable electrostatics from an arrayof inorganic anions, affords MOFs with unprecedented CO₂ uptake andselectivity in the context of bulk (5% and higher) CO₂ capture. Herein,we report how this same approach can be used to develop isostructuralmaterials rather suitable to air capture and traces CO₂ removal. Incontrast to amine scrubbing systems, amine supported materials, andsodalime sorbents, these materials exhibit also very high butnon-reactive and uniformly distributed CO₂ energetics, pushing theborders in enhancement of physical interactions in MOFs, in addition tofully reversible physical driven adsorption-desorption operations atvery mild condition. This work shows that due to their ability forrational pore size modification and inorganic-organics moietiessubstitution, MOFs with periodically arrayed hexaflourosilicate (SIFSIX)pillars offers for the first time remarkable CO₂ adsorption, uptake andselectivity in highly diluted gas streams that other plain class ofmaterials are unable to achieve.

Introduction/Discussion:

The growing interest in combatting the greenhouse gas effect¹ triggereda rising interest in the direct air capture (DAC) as a viable option toreduce greenhouse gases emissions in a uniform way.²⁻⁵ Although morechallenging than post-combustion capture, it is recognized that DACmight be feasible, provided that suitable adsorbent combining optimumuptake, kinetics, energetics and CO₂ selectivity is available at tracesCO₂ concentration.⁶

Particularly the removal of traces of CO₂ from air for industrialpurposes is a growing area of research and development, owing to itssubstantial importance particularly for pre-purification of air, whenatmospheric air is used during the separation of nitrogen and oxygen. Infact, prior to air separation using cryogenic distillation or pressureswing adsorption (PSA), air must be CO₂ free to avoid (i) blockage ofheat-exchange equipment as a result of frozen CO₂ during theliquefaction process^(7,8) and (ii) adsorbents (e.g., zeolites)contamination used for oxygen production by PSA.⁹

At the same level of importance, alkaline fuel cells (AFCs) requireoxygen and hydrogen used as feedstock to be CO₂ free. Indeed, traceamounts of CO₂ (300 ppm) degrade the electrolyte in AFCs.¹⁰⁻¹² Inaddition, humans require oxygen, and in return emit carbon dioxide.Therefore, efficient removal of CO₂ at low concentrations is also vitalfor the proper operation of breathing systems in confined spaces such assubmarines and aerospace shuttles.¹³⁻¹⁵ In fact, in long-term spaceflight and submarine missions, CO₂ must be removed from the air andrecycled because resupply opportunities are less frequent ornon-existent. An average crew member requires approximately 0.84 kg ofoxygen and emits approximately 1 kg of carbon dioxide.¹⁵ Thus theability to continuously purify the exhaled air (with a maximum CO₂concentration of 2-5%) will lead to an optimal recycling andconsiderable reduction in fresh air supply in closed, confined spaces.

Efficient CO₂ removal and resupply of fresh air is also of primeimportance in mining and rescue missions,¹⁶ diving, and most importantlyin medical applications such as anaesthesia machines.¹⁷ The use ofanaesthesia machine was and still a growing clinical trend throughoutthe world, driven by the need to reduce cost and improve patient carevia the use of efficient CO₂ sorbents. CO₂ removal feature inanaesthesia machine is particularly important in semi-closed or closedrebreathing systems, as the rebreathing fraction is at least 50% of theexhaled gas volume, directed back to the patient after proper CO₂removal in the next exhalation. Sodalime is currently the sorbent ofchoice in most commercially available anaesthesia machines. This sorbentexhibits a high CO₂ removal efficiency from exhaled air, with an averagecontinuous operation of about 24 hours using a pre-packed commercialcartridge.¹⁸ Nevertheless, a major drawback of this technology is thatone sodalime cartridge can only be used for a single cycle and isnon-regenerable, generating therefore a huge amount of waste that shouldbe disposed properly. Recently, a growing interest to low CO₂concentration removal applications,¹² was spotted and few materials werereported to adsorb efficiently traces of CO₂, particularly with regardsto DAC using a variety of amine supported (silica based)materials.^(12,19) Recently, metal-organic frameworks (MOFs), which is aburgeoning class of porous materials, was intensively investigated forintermediate and high CO₂ concentration removal applications such aspost-combustion, pre-combustion capture, natural gas and biogasupgrading.²⁰⁻²³ However, the capability of MOFs to remove traces and lowCO₂ concentration from gas streams was rarely debated.²⁴⁻²⁷ The mainreason for this lack of studies is that most of MOFs reported so farwith or without unsaturated metal sites (UMC) or/and functionalizedligands exhibited relatively low selectivity and uptake particularly atlow CO₂ partial pressure. With the aim to enhance the CO₂ adsorptionenergetics and uptake in MOFs and covalent organic frameworks (COFs), afew scientist,^(26,27) were inspired by the amine chemistry and the hugeknow how gained so far from amine-supported silica.²⁰⁻²² In fact,recently, Jones and co-workers studied for the first time the effect ofethylenediamine (ED) grafting with Mg-MOF-74 as a support for CO₂adsorption from ultra-dilute gas streams such as ambient air.²⁸ Long andco-workers investigated the effect of N,N-dimethylethylenediaminegrafting for DAC using an expanded version of Mg-MOF-74.²⁸ Thus, the fewstrategies reported so far targeting air capture using MOFs rely on theability of grafted amines to form strong chemical bonding (at least 70kJ/mol) with CO₂, affording high affinity toward CO₂ and therefore highCO₂ selectivity. Interestingly, there is no work reported so far ontuning pore size of plain MOFs with the optimal CO₂ energetics (strong,uniform and enough low to allow reversible physicaladsorption-desorption) to target traces CO₂ removal in general and DACapplication in particular.

Results and Discussion:

Recently, we reported a CO₂ study on a series of isoreticular MOFs withperiodically arrayed hexaflourosilicate (SIFSIX) pillars, calledSIFSIX-2-Cu-i and SIFSIX-3-Zn (FIG. 1). These porous MOFs having acombination of tunable pore size (rather than large surface area)coupled with requisite chemistry led to materials exhibiting fast andhighly selective CO₂ behaviour over N₂, CH₄ and H₂ with uniformlyaligned strong CO₂ adsorption sites.²³ Particularly the denserisoreticular analogue of SIFSIX pillars; SIFSIX-3-Zn revealed verysteeper variable CO₂ adsorption isotherms (FIG. 12) than SIFSIX-2-Cu-I(FIG. 11) suitable for post-combustion capture (at the CO₂ partialpressure of 100-mbar), but also excellent features suitable for naturaland biogas upgrading, as well as pre-combustion capture^([20]) (high CO₂concentration and high pressure).

Because of the importance of this discovery and the unprecedented steepCO₂ adsorption isotherms over a wide range of temperature, particularlyfor this class of materials; MOFs, we found it compelling to explore thecapability of these SIFSIX MOFs for CO₂ adsorption in traceconcentration (diluted streams in vacuum or in mixture containing largefraction of N₂ up to 95%). In order to highlight the concealedcapability of these MOFs for low CO₂ concentration related applicationsinvolving CO₂ concentration below 5% (below 50 mbar CO₂ partialpressure) such as anaesthesia machines, pre-purification before airseparation and air capture, single gas CO₂ adsorption was investigatedfor SIFSIX-2-Cu-i and SIFSIX-3-Zn. Upon contraction the pore size from5.15 Å (for SIFSIX-2-Cu-i) to 3.8 Å (for SIFSIX-3-Zn) the CO₂ uptakeincreased drastically (FIG. 13) resulting to the highest CO₂ uptake everreported for MOFs in the range of below 5% CO₂. For example, SIFSIX-3-Znshowed an order of magnitude higher volumetric CO₂ uptake (55 cm³(STP)/cm³) than Mg-MOF-74,²⁰ (28 cm³ (STP)/cm³) at 10 mbar (1% CO₂),while UTSA-16,²⁹ exhibited much lower CO₂ uptake similar toSIFSIX-2-Cu-I.

To further investigate the effect of tuning further the pore size on theadsorption properties of SIFSIX-3-M, and by studying other coordinationpolymers constructed from hexaflourosilicate ions with pyrazine, wefound that the bonding of the Cu(II) with pyrazine leads to a slightlyshorter M-N(nitrogen) bond than the zinc (see table Si in SI),³⁰ whichwill lead to further decrease in the pore size of the constructed 3DMOF, if we could substitute Zn by Cu. In order to explore this prospect,we deliberately intended to prepare the SIFSIX-3-Cu analogue which wasnever reported before and it was successfully prepared by layering amethanol solution (5.0 mL) of pyrazine (pyz, 0.30 g, 3.0 mmol) in aglass tube onto a methanol solution (5.0 mL) of CuSiF₆·xH₂O (0.325 g,0.6 mmol). Upon layering, an extremely fast formation of light violetpowder was observed, and the powder was left for 24 hours in the mothersolution. The powder was then collected and washed extensively withmethanol then dried under vacuum and characterized using powder X-raydiffraction (PXRD).

The PXRD diagram was found not to match with any of the related reportedstructures, i.e. the Cu-2D structures or the 3D Zn analogue reported byKita et al.³⁰ (FIG. 5). Despite extensive attempts, it was not possibleto isolate synthetic conditions affording single crystals of sufficientsize for single crystal diffraction (SCD), and the structure was thensolved from PXRD using direct methods. The structural model was thenenergetically and geometrically refined, and the good agreement betweenexperimental and calculated PXRD diagrams validates our model (FIGS. 6and 7). Analysis of the structure revealed the formula of the Cuanalogue; [Cu(SiF₆)(pyz)₂.solv]. As initially expected, it is in verygood agreement with the 3D structure of the Zn analogue reportedpreviously (FIG. 1),³⁰ but with a slightly smaller unit cell (375 vs.388 Å³) attributed to the stronger bonding between the Cu(II) and thepyrazine (Table S1 in SI).³⁰ The smaller unit cell of the Cu analoguewas in good agreement with the relatively sharp pore size distribution(PSD) analysis centred at 3.5 Å (average pore size), as determined fromthe CO₂ isotherms, which shows smaller average pore size than theSIFSIX-3-Zn (3.5-4 Å) (FIG. 2(right)). The thermal gravimetric analysis(TGA) of the SIFSIX-3-Cu was tested in the temperature range 25-250° C.The thermogram (FIG. 8) shows a mass loss of about 10% for the driedsample in the range of 50-150 corresponding to guest molecules. Furthergradual loss was observed above 150° C. due to the decomposition andloss of pyrazine and SiF6 ions. The TGA for the SIFSIX-3-Cu is in a goodagreement with the one reported for the SIFSIX-3-Zn.³⁰ Infra-red (IR)spectrum for the SIFSIX-3-Cu (FIG. 9), exhibits bands characteristic ofthe C—H aromatic bonds of the pyrazine at 3114 and 3073 cm⁻¹ and bandscharacteristic for the C-N bond at 1445, 1122 and 1070 cm⁻¹. In additionto that the characteristic bands for the octahedral SiF6 were alsoobserved at 743 and 833 cm⁻¹.³¹

It is logically expected that this new Cu analogue should at least showthe same promising adsorption properties as the SIFSIX-3-Zn.²³Surprisingly, the Cu analogue showed even steeper variable temperatureadsorption isotherms (FIG. 2(left)) at very low pressure indicative ofthe stronger CO₂-SIFSIX-3-Cu interactions.

The mechanistic behind the unprecedented selective CO₂ adsorptioninvolving the unique synergetic effect of thermodynamics and kinetics²³was confirmed by the competitive kinetics of CO₂/N₂: 10/90 gas mixtureadsorption (FIG. 14). As was expected and based on the similar studycarried out on the Zn analogue,²³ the uptake at equal times for variableCO₂ compositions mixtures follow the behaviour of pure CO₂ (FIG. 10). Inaddition at equilibrium, the total uptake of the CO₂ containing gasmixtures overlay perfectly with the equilibrium uptake for pure CO₂(FIG. 10). These findings show that similarly to SIFSIX-3-Zn, when CO₂containing mixtures are in contact with SIFSIX-3-Cu, CO₂ adsorbs morestrongly and faster than N₂ (and by analogy also O₂, CH₄ and H₂ (FIG.14, thus occupying all the available space and sorption sites andexcluding other gases which is a desirable feature in many CO₂separation and purification applications. Examination of the SIFSIX-3-Madsorption results in the spectra of low concentration applications (400ppm-5%) showed that the Cu analogue exhibits even steeper adsorptionisotherms at very low CO₂ concentration (FIG. 3a ) translated into thehighest uptake ever reported for MOFs without UMC or amino groups at lowCO₂ pressure below 38 torr (0.05 bar). This unprecedented finding iseven more interesting owing to its fully physical adsorption naturewhere complete desorption of CO₂ was established at only 323 K. At 7.6torr (0.01 bar) SIFSIX-3-Cu exhibited 82.6 cm³(STP)/cm³ vs. 55 and 28cm³(STP)/cm³ for SIFSIX-3-Zn and Mg-MOF-74, respectively. Interestingly,the gravimetric uptake of SIFSIX-3-Cu at 400 ppm and 298 K (1.24 mmol/g)is 10 and 15.5 times higher than the corresponding uptakes forSIFSIX-3-Zn (0.13 mmol/g) and Mg-MOF-74 (0.08 mmol/g) and even higherthan the uptake of most of amine-supported silica materials (withoptimal compromise of amine loading and kinetics)³² at 298 K (forexample TRI-PE-MCM-4¹¹⁻²³ (1 mmol/g)). Table 1 shows a summary of theadsorption uptake at variable low CO₂ concentration (partial pressures)for SIFSIX compounds as compared to Mg-MOF-74 and amine supportedmaterials (including MOFs); relevant to different traces CO₂ removalapplications. It is to notice that SIFSIX-3-Cu showed even higher CO₂uptake at 400 ppm and 328 K as compared to the corresponding uptake at323 K for amine functionalized Mg-dobpdc-mmen (Table S3).

TABLE 1 ((CO₂ adsorption uptake at various traces CO₂ concentration andat 298 K in comparison to various amine supported materials)) uptake at400 ppm Uptake at 5000 ppm Uptake at CO₂ Qst Adsorbent (0.4 mbar) (5mbar) 10000 ppm (10 mbar) (kJ/mol) SIFSIX-2-Cu-i 0.0684^(c)/0.2^(d)0.097^(c)/2.7^(d ) 0.19^(c)/5.32^(d) 32 SIFSIX-3-Zn  0.13^(c)/5.6^(d) 1.12^(c)/39.26^(d)  1.53^(c)/53.97^(d) 45 SIFSIX-3-Cu 1.24^(c)/43.9^(d) 2.26^(c)/79.8^(d) 2.34^(c)/82.5^(d) 54 Mg-MOF-740.088^(c)/1.8^(d)  0.7^(c)/14.3^(d)  1.27^(c)/25.86^(d) 47Mg-MOF-74-ED^(a) 1.5^(c) ND ND ND Mg-dobpdc-mmen^(b) 2^(c  ) 2.5^(c )2.75^(c) 70 TRI-PE-MCM-41^(f) 1^(c  ) 1.45^(c) 1.6^(c ) 92 HAS^(f)1.7^(c) ND ND ND ^(a)((Ethylenediamine functionnalized²⁴));^(b)((N,N-dimethylethylenediamine functionnalized²⁵)); ^(c)mmol/g;^(d)cm³ (STP)/cm³; ^(e)at 328 K; ^(f)Amine supported silica. ND: nondetermined;

Interestingly, upon the substitution of Zn by Cu, the Q_(st) of CO₂adsorption in the contracted structure increased by 20%, from 45 to 54kJ mol⁻¹ (FIG. 3b ), in perfect agreement with the relatively steeperCO₂ adsorption isotherms in case of the Cu analogue at very lowpressure. This increase is mainly attributed to the small unit cell andthe small pore size of the Cu analogue. The Q_(st) of CO₂ adsorption isan intrinsic property that dictates the affinity of the pore surfacetoward CO₂; this in turn plays a major role in determining theadsorption selectivity and the necessary energy to release CO₂ duringthe regeneration step. Although the Q_(st) for CO₂ was slightly abovethe range of fully reversible CO₂ adsorption (30-50 kJ mol⁻¹)²³,SIFSIX-3-Cu was fully evacuated at 50° C. in vacuum (or in N₂environment). As in case of SIFSIX-3-Zn and SIFSIX-2-Cu-i, the Q_(st)for CO₂ adsorption was relatively constant up to high CO₂ loadingsindicating homogenous binding sites over the full range of CO₂ loading(FIG. 3b ).²³

The CO₂ selectivity of SIFSIX-3-Zn and SIFSIX-3-Cu was investigatedexperimentally using column breakthrough tests for binary CO₂/N₂:1000ppm/99.99% mixture (FIG. 4 left) at 298 K in dry as well as in humidconditions. In dry condition, the first CO₂ signal downstream the columnwas observed only after ca. 798 and ca.1922 min/g for SIFSIX-3-Zn andSIFSIX-3-Cu, respectively after starting continuous CO₂/N₂ gas mixtureflux (5 cm³min⁻¹), while N₂ breakthrough occurred in a few seconds.Accordingly, at 1000 ppm CO₂ and breakthrough time, SIFSIX-3-Cu showedhigher selectivity (ca. 10500) than SIFSIX-3-Zn (7259). We also notethat the calculated and measured selectivity exceeding 1000-2000 areoften subject to uncertainties associated with measurement of the gasuptake of weakly adsorbed gases (N₂) in the mixture, thus the reportedselectivity is highly qualitative and aiming the comparison of thestudied compounds only.

The CO₂ removal selectivity at 1000 ppm CO₂ on SIFSIX-3-Cu was notaffected by the presence of humidity as shown from the columnbreakthrough tests performed on both compounds at the relative humidity(RH) of 74% (FIG. 4(right)). This unprecedented finding was also validin case of SIFSIX-3-Zn for the removal of low (FIG. 15) and higher CO₂concentration.²³

In conclusion, we showed herein how a material design and engineeringapproaches to pore size control in combination with suitable energeticsof favourable electrostatics from an array of inorganic anions affordsMOFs with unprecedented CO₂ uptake and selectivity in the context of aircapture and traces CO₂ removal. These materials exhibit very high(non-reactive) CO₂ energetics but fully reversible physical drivenadsorption-desorption operations at very mild conditions, without thewell documented drawbacks associated with amine reactive chemistry.

This work shows for the first time that thanks to their ability forrational pore size modification and inorganic-organics moietiessubstitution, MOFs offers remarkable CO₂ adsorption attributes in highlydiluted gas streams that other plain class of materials are unable toachieve. Further works will be dedicated to study the effect ofsubstituting other metals such as cadmium, cobalt, chromium etc., on theCO₂ separation properties in diluted CO₂-containing gases.

Methods: SIFSIX-3-Cu:

A methanol solution (5.0 mL) of pyrazine (pyz, 0.30 g, 3.0 mmol) waslayered in a glass tube onto a methanol solution (5.0 mL) of CuSiF₆·xH₂O(0.325 g, 0.6 mmol). Upon layering, a fast formation of light violetpowder was observed, and the powder was left for 24 hours in the mothersolution. The powder was then collected and washed extensively withmethanol then dried under vacuum Characterization:

The powder PXRD patterns were recorded on a Panalytical X'pert PRO MPDX-ray Diffractometer with Cu Kα radiation (λ=0.15418 nm, 45 kV, 40 mA).

Low pressure gas sorption measurements were performed on a fullyautomated micropore gas analyzer Autosorb-1C (Quantachrome Instruments)at relative pressures up to 1 atm. The cryogenic temperatures werecontrolled using a liquid nitrogen bath at 77 K. Pore size analyses wereperformed using a cylindrical NLDFT pore model system by assuming anoxidic (zeolitic) surface.

High Pressure Adsorption:

Adsorption equilibrium measurements of pure gases were performed using aRubotherm gravimetric-densimetric apparatus (Bochum, Germany) (Scheme51), composed mainly of a magnetic suspension balance (MSB) and anetwork of valves, mass flowmeters and temperature and pressure sensors.The MSB overcomes the disadvantages of other commercially availablegravimetric instruments by separating the sensitive microbalance fromthe sample and the measuring atmosphere and is able to performadsorption measurements across a wide pressure range, i.e. from 0 to 20MPa. The adsorption temperature may also be controlled within the rangeof 77 K to 423 K. In a typical adsorption experiment, the adsorbent isprecisely weighed and placed in a basket suspended by a permanent magnetthrough an electromagnet. The cell in which the basket is housed is thenclosed and vacuum or high pressure is applied. The gravimetric methodallows the direct measurement of the reduced gas adsorbed amount Q.Correction for the buoyancy effect is required to determine the excessand absolute adsorbed amount using equation 1 and 2, where Vadsorbentand Vss and Vadsorbed phase refer to the volume of the adsorbent, thevolume of the suspension system and the volume of the adsorbed phase,respectively.Ω=m _(absolute)−ρ_(gas)(V _(adsorbent) +V _(ss) +V_(adsorbed-phase))  (1)Ω=m _(excess)−ρ_(gas)(V _(adsorbent) +V _(ss))  (2)

The buoyancy effect resulted from the adsorbed phase maybe taken intoaccount via correlation with the pore volume or with the theoreticaldensity of the sample.

Structure Determination

DICVOL06 was used for pattern indexing of SIFSIX-3-Cu; the cellrefinement was carried out by a structureless whole pattern profilerefinement using the FullProf software and its graphical interfaceWinPlotr. (J. Epdic 7: European Powder Diffraction, Pts 1 and 2 Vol.378-3 Materials Science Forum (eds R. Delhez & E. J. Mittemeijer)118-123 (2001)) The structure of SIFSIX-3-Cu was solved ab initio on theas-synthesized solid using powder X-Ray diffraction (PXRD) data bydirect method using Expo2009. (J. Appl. Crystallogr. 42, 1197-1202,(2009)). All framework atoms were found directly and their coordinatesgeometrically and energetically refined through Forcite in MaterialsStudio 6.0.0.

TABLE S1 Comparison of characteristic interatomic distances in SIFSIXbased MOFs d_(M-N) d_(M-F1) D_(Si-F1) D_(Si-F2) MOF (Å) (Å) (Å) (Å)†reference Ligand SIFSIX-1-Zn   SIFSIX-1-Cu     2.157 2.131 2.007 2.0091.966 2.082 2.122 2.379 2.357 2.336 1.757 1.720 1.703 1.698 1.692 1.6001.650 1.672 1.609 1.685 ³ ⁴ ⁵ ⁶ ⁷

SIFSIX-2-Zn SIFSIX-2-Cu SIFSIX-2i-Cu 2.125 2.027 2.015 2.069 2.300 2.3531.698 1.684 1.693 1.668 1.684 1.679 ⁴ ⁸ ⁸

SIFSIX-3-Zn SIFSIX-3-Cu SIFSIX-2D-Cu SIFSIX-2D-H₂O—Cu 2.172 2.046 2.061* 2.031 2.057 2.259 2.402 2.412 1.747 1.684 1.727 1.695 1.6571.686 1.688 1.675 ⁹ this work ⁹ ⁷

SIFSIX-4-Zn 2.117 2.062 1.712 1.653 ⁴

SIFSIX-5-Cu 2.012 2.258 1.727 1.681 ⁵

†Average distance, due to disorder of F atoms. *Average distance, due tolower symmetry, SIFSIX-2D contains two independent N atoms bonded to Cu³ Subramanian, S. & Zaworotko, M. J. Porous Solids byDesign-[Zn(4,4′-Bpy)(2)(Sif6)](N)Center-Dot-Xdmf, a Single FrameworkOctahedral Coordination Polymer with Large Square Channels (Vol 34, Pg2127, 1995). Angew. Chem. Int. Ed. 34, 2127-2129, (1995). ⁴ Lin, M.-J.,Jouaiti, A., Kyritsakas, N. & Hosseini, M. W. Molecular tectonics:modulation of size and shape of cuboid 3-D coordination networks.Crystengcomm 11, 189-191, (2009). ⁵ Burd, S. D. et al. Highly SelectiveCarbon Dioxide Uptake by Cu(bpy-n)2(SiF6)(bpy-1=4,4′-Bipyridine;bpy-2=1,2-Bis(4-pyridyl)ethene). J. Am. Chem. Soc. 134, 3663-3666,(2012). ⁶ Noro, S., Kitagawa, S., Kondo, M. & Seki, K. A new, methaneadsorbent, porous coordination polymer[{CuSiF6(4,4′-bipyridine)(2)}(n)]. Angew. Chem. Int. Ed. 39, 2082-+,(2000). ⁷ Noro, S. et al. Framework engineering by anions and porousfunctionalities of Cu(II)/4,4′-bpy coordination polymers. J. Am. Chem.Soc. 124, 2568-2583, (2002). ⁸ Nugent, P. et al. Porous materials withoptimal adsorption thermodynamics and kinetics for CO2 separation.Nature 495, 80-84, (2013). ⁹ Uemura, K., Maeda, A., Maji, T. K., Kanoo,P. & Kita, H. Syntheses, Crystal Structures and Adsorption Properties ofUltramicroporous Coordination Polymers Constructed fromHexafluorosilicate Ions and Pyrazine. Eur. J. Inorg. Chem., 2329-2337,(2009).

TABLE S2 Crystallographic data of SIFSIX-3-Cu. Compound SIFSIX-3-CuFormula (dried solid) C₈N₄H₈CuSiF₆ Molar weight (g.mol⁻¹) 365.80Calculated density (g.cm⁻³) (dried solid) 1.62 Symmetry Tetragonal Spacegroup P 4/m m m (n^(o) 123) a (Å) 6.901(1) b (Å) 6.901(1) c (Å) 7.886(1)V (Å³) 375.5 Z 1 Wavelength λ(Cu K□) 1.5406 Temperature (K) 298 Angularrange 2-theta (°) 3-80 Number of independent atoms (dried solid) 8

TABLE S3 Gravimetric CO₂ uptake at 400 ppm and 328 K for SiF6—Cu-3,SiF6—Zn-3 and SiF6—Cu-2-i in comparison to Mg-dobpdc-mmen Uptake at 400ppm (0.4 mbar) Adsorbent mmm/g SIF6-2-Cu-i negligible SIF6-3-Zn 0.0287SIF6-3-Cu 0.242 Mg-dobpdc- ≈ 0.12 mmen^(b) ^(b)at 323 K

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Microporous metal-organic framework with    potential for carbon dioxide capture at ambient conditions. Nature    Communications 3, doi:10.1038/ncomms1956 (2012).-   30 Uemura, K., Maeda, A., Maji, T. K., Kanoo, P. & Kita, H. Crystal    structures and adsorption properties of ultramicroporous    coordiunation polymers constructed ions from hexafluorosilicate ions    and pyrazine. Eur. J. inorg. Chem., 2329-2337 (2009).-   31 Conley, B. D., Yearwood, B. C., Parkin, S. & Atwood, D. A.    Ammonium hexaflurosilicate salts. Journal of Fluorine Chemistry 115,    155-160 (2002).-   32 It is important to notice that hyperbranched aminosilica material    (HAS)25 was reported to exhibit higher gravimetric uptake (1.7    mmol/g) at 390 ppm and 298 K, however the kinetics of adsorption was    much slower due the diffusion limitation caused by the high amine    loading. 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In regard to the discussion herein including the Examples above and theclaims, it should be noted that ratios, concentrations, amounts, andother numerical data may be expressed herein in a range format. It is tobe understood that such a range format is used for convenience andbrevity, and thus, should be interpreted in a flexible manner to includenot only the numerical values explicitly recited as the limits of therange, but also to include all the individual numerical values orsub-ranges encompassed within that range as if each numerical value andsub-range is explicitly recited. To illustrate, a concentration range of“about 0.1% to about 5%” should be interpreted to include not only theexplicitly recited concentration of about 0.1 wt % to about 5 wt %, butalso include individual concentrations (e.g., 1%, 2%, 3%, and 4%) andthe sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within theindicated range. In an embodiment, the term “about” can includetraditional rounding according to measurement techniques and the unitsof the numerical value. In addition, the phrase “about ‘x’ to ‘y’”includes “about ‘x’ to about ‘y’”.

Many variations and modifications may be made to the above-describedembodiments. All such modifications and variations are intended to beincluded herein within the scope of this disclosure and protected by thefollowing claims.

We claim:
 1. A system for capturing CO₂ in a gas, comprising: a firststructure including a metal-organic material (MOM), wherein the MOMincludes at least one of an inorganic linker or an inorganic pillar,wherein the gas includes CO₂ and water vapor, wherein the MOM has agreater relative affinity for CO₂ over a water; and a second structurefor introducing the gas to the first structure, wherein the firststructure is configured to remove CO₂ from the gas to form a modifiedgas, wherein the second structure is configured to flow the modified gasaway from the first structure so the CO₂ is captured in the firststructure to the exclusion of the modified gas.
 2. The system of claim1, wherein the gas includes at least one of the following gases: N₂, H₂,and CH₄, wherein the MOM has a greater relative affinity for CO₂ overeach one of N₂, H₂, and CH₄.
 3. The system of claim 1, wherein the MOMis [Cu(pyr)₂(SiF₆)]_(n), wherein n is 1 to 100,000,000, and wherein theMOM has a pore size of about 3.35 Å.
 4. The system of claim 1, whereinthe MOM is a hydrophobic MOM.
 5. The system of claim 1, wherein the MOMis selected from the group consisting of:[Cu(4,4′-dipyridylacetylene)₂(SiF₆)]_(n), where n is 1 to 100,000,000; apair of interpenetrated nets of[Cu(4,4′-dipyridylacetylene)₂(SiF₆)]_(n); and [Zn(pyr)₂(SiF₆)]_(n),wherein n is 1 to 100,000,000.
 6. The system of claim 1, wherein the MOMhas a primitive cubic topology.
 7. The system of claim 1, wherein thegas includes CO₂ at a concentration of 5% or less.